The present invention is generally related to control of electric machines, and, more particularly, to a controller device and method for providing current control of power converter driven electric machines.
To overcome the disadvantages of DC motors in DC powered applications, it is known to use synchronous machines in either the form of a permanent magnet (PMAC) wound field or a synchronous reluctance machine (SyR) or induction machine (asynchronous) in conjunction with a power inverter. However, such motors typically exhibit nonlinear characteristics and parameter variations due to temperature, magnetic saturation, or both, when operated over a wide operating range. In addition, the variation of drive input values, such as battery voltage, can negatively affect the motor performance and can cause loss of motor control. Accordingly, the use of known vector control techniques, coupled with a pulse width modulation (PWM) strategy and a power inverter, allows the flux producing component and torque producing component of motor current to be decoupled to produce a motor response analogous to that of a DC motor. With the rapid advancement of smaller and faster processors, vector control techniques have become practical for control of synchronous and asynchronous machines.
PMAC machines can be designed to posses a significant field-weakened region. This design approach reduces the inverter size by reducing the required current per phase that correspondingly reduces the drive unit size and cost. The controller, however, must become more sophisticated in order to properly control the drive to maintain high efficiency and acceptable dynamic performance in the field-weakened range. The most efficient operation for a PMAC above its base speed is achieved when the smallest amount of current is used to weaken, or de-flux, the magnetic flux of the magnet. This condition results in operation where the controller operates the machine near the voltage limit of the system. If the available voltage to the inverter is changed or the magnetic flux is varied due to temperature change, the amount of field-weakening current should be adjusted in order to maintain control and high efficiency. The control should also be modified from the traditional approach in order to increase the drive performance and improve stability for the voltage limited condition. The dynamics of the control may be of paramount importance since operation in the voltage limited condition could cause certain undesirable effects, such as voltage saturation, that can cause slow response or loss of machine control.
Synchronous reluctance machines can also be designed to have a significant field-weakened range. Unlike the PMAC, this machine does not have an already established field flux in the form of a permanent magnet, so the torque produced is from reluctance torque. In the field-weakened range, the torque and efficiency are limited by the available bus voltage. If the field can be increased without exceeding a voltage limit, greater efficiency and higher torque can be produced from the machine at a given operating point.
Similarly, induction machines can be designed to have a significant field-weakening range. Like the synchronous reluctance machines, induction machines do not have an already established field flux in the form of a permanent magnet, so the flux has to be built by an Ids (d-axis synchronous) current. Accordingly, the torque produced is a product of Iqs (q-axis synchronous) current and flux. In the field-weakened range, the torque and efficiency are limited by the available bus voltage. If the field flux can be increased without exceeding a voltage limit, greater efficiency and higher torque can be produced from the machine at a given operation point.
Thus, means for setting the flux as a function of operating conditions is desired. If too little current is used to weaken the field for a PMAC machine or, analogously, if too much flux current is used in a SyR machine or an induction machine, then control can be lost due to voltage limit conditions. In contrast, if too much current is provided in a PMAC machine or, analogously if too little flux current is used in a SyR machine or an induction machine, then the efficiency will be too low. Importantly, dynamically changing operational parameters, such as varying magnetic characteristics of the machine as the rotor temperature changes or DC supply voltage changes, must be dynamically accounted for to protect against a loss of control of the machine. While look-up tables have been proposed to perform this function, the values stored in the table may not be valid under dynamically changing operating condition.
Accordingly, it would be desirable to provide flux control in response to changing operating conditions in synchronous and asynchronous machines. In particular, it would be desirable to provide dynamic flux control in the field-weakening region of machine operation. It would be further desirable to make the foregoing technique substantially impervious to variations in battery voltage and operating temperature, and parameter variations from machine to machine.